1. Field of the Invention
This invention relates generally to a wireless-to-wireline and wireless-to-wireless communication system that is composed of wireless access networks interconnected via a wireline IP (Internet Protocol) network, and, more particularly, to methodologies and concomitant circuitry for effecting soft handoff in the wireless portion of the system.
2. Description of the Background Art
Today, many different wireless systems exist, ranging from indoor wireless LANs (Local Area Networks) to outdoor cellular systems. Generally, the numerous wireless systems are not compatible with each other, making it difficult to roam from one system to another. Although there have been attempts to unify third-generation wireless systems, incompatible systems are expected to co-exist in the future. Furthermore, wireless LANs and cellular wireless systems are being developed independently, and such systems are also evolving independently. So far, no wireless technology has emerged as a common and long-term universal solution.
IP (Internet Protocol), which is already a universal network-layer protocol for packet networks, is rapidly becoming a promising universal network-layer protocol for wireless systems. An IP terminal, with multiple radio interfaces, can roam between different wireless systems if they all support IP as a common network layer. Unlike today's wireless systems in which Radio Access Networks (RANs) are mostly proprietary, IP provides an open interface and promotes an open market. IP will also enable widely adopted and rapidly growing IP-based applications to run over wireless networks. Moreover, distributed, autonomous IP-based wireless base stations have the potential of making the wireless systems more robust, scalable, and cost effective.
There are, however, many challenges to realizing distributed all-IP wireless networks. For the sake of specificity in discussing these challenges as well as pointing out problem areas, reference is made to FIG. 1. The depiction of network 100 in FIG. 1 illustrates an exemplary configuration of a network that uses IP-based wireless base stations (designated iBSs). The coverage area of the wireless network is defined by a multiplicity of cells (e.g., cells 101, 102, 103). The geographical area covered by each wireless base station is referred to as a cell (e.g., iBS 111 serves cell 101, and so forth). When mobile station 104 moves from one cell (e.g., cell 101 originally) into the overlapping regions (e.g., overlap of cells 101 and 102) of the coverage areas of multiple base stations, base station 111 may perform a “handoff” of mobile station 104 to base station 112. Handoff is a process whereby a mobile station communicating with one wireless base station is switched to another base station during a session. Overlap regions 117 and 118 are coverage areas where handoff is effected. For example, as mobile station 104 moves into region 117 while roaming in cell 101, the radio signal strength from iBS 2 (depicted by reference numeral 115) may be greater than the radio signal strength (114) from iBS 1, so handoff is warranted to maintain the quality of the established session.
Among the key challenges in a distributed all-IP wireless network is how to support “soft handoff”. As suggested above, handoff is the process that allows a mobile station's session-in-progress to continue without interruption when a mobile station (MS) moves from one wireless cell to another. Soft handoff is a form of handoff whereby a mobile station can start communication with the target base stations without interrupting the communication with the serving base station. Thus, soft handoff allows a MS to communicate with multiple base stations (BSs) simultaneously. In particular, soft handoff has been shown to be an effective way for increasing the capacity, reliability, and coverage range of CDMA-based wireless networks. Soft handoff also provides more time for carrying out the handoff procedure.
Soft handoff in a CDMA-based wireless system is the focus of the subject matter of the present invention. In Code Division Multiple Access (CDMA) radio systems, a narrowband user message signal is multiplied by a very large bandwidth signal called the spreading signal. The spreading signal is a pseudo-noise code sequence that has a communication signal rate which is orders of magnitudes greater than the data rate of the user message signal. All users in a CDMA system may transmit simultaneously. Each user has its own pseudorandom code for coding its own message signal—each code is approximately orthogonal to all other codes. A receiver is assigned a code to detect a desired user message signal, and performs a time correlation operation to detect only the specific assigned code. All other codes appear as noise due to de-correlation. CDMA is effective in wireless systems because a receiver can be assigned a multiplicity of codes to detect message signals from a corresponding multiplicity of transmitters, thereby engendering the soft handoff process.
An IP router is an IP network device that runs IP layer routing protocol (e.g., OSPF and BGP) and forwards IP packets. The running of a routing protocol decides the “routing policy”, and the forwarding of IP packets realizes the “routing mechanism”. IP packets arriving from the wireline IP network (121) at a given base station (e.g., iBS 111 over wireline path 122 or iBS 112 over path 123) can be routed by the routing mechanism of the base station to mobile station 104 (or other appropriate wireline devices that connect directly to the base station).
Today, the only known approach to designing an IP-based base station is to add (or connect) radio transmission and receiving equipment directly onto an IP router (131). Such a design, however, has a potentially serious shortcoming. In particular, the mobile stations served by different base stations must belong to different IP subnets, that is, the design forces the mobile stations in different cells to be on different IP subnets. (Here, a subnet is used in the sense defined by an IP address, which has the form, for example, “w.x.y.z” (e.g., 129.3.2.14), wherein “w.x” is the network address (129.3), “y” (2) is the subnet address for a device associated with the given network, and “z” (14) is the host address for a device associated with the given network/subnet, such as a mobile terminal or a base station. In terms of FIG. 1, iBS 1 may be assigned the subnet address 2, whereas mobile station 104 may have the host address 14.) Suppose, for the sake of argument, that a mobile station is served by two base stations belonging to the same IP subnet S. Then, both iBSs (IP routers) will advertise to other routers in the overall network that they can reach all the hosts on subnet S. However, each iBS can only reach a subset of the hosts on subnet S (i.e., the set of hosts being currently served by the base station). This means that other routers will not be able to determine which base station should receive a packet destined for a host on subnet S. In other words, packets may be delivered to the wrong base station and consequently cannot reach the destined host.
The fact that mobile stations (MSs) in different cells belong to different IP subnets suggests that an MS may have to change its IP address every time it moves into a new cell. Changing IP address usually takes a long time using today's methods for dynamic IP address assignment (e.g., the Dynamic Host Configuration Protocol or DHCP). When certain IP-layer mobility management mechanisms are used (e.g., SIP-based mobility management), a change of IP address can also mean that the old session may need to be modified, or new SIP sessions may have to be established.
Having to change IP addresses when moving from one cell to another also makes soft handoff more difficult to implement. For example, if an MS has to use different IP addresses to receive IP packets from different iBSs, IP packets coming to the MS from different iBSs will not be identical because they carry different IP destination addresses. Consequently, copies of the same packets from different base stations may not be correctly combined by the MS's radio system.
Recently, methods (e.g., HAWAII, Cellular IP) have been proposed to enable MSs to move within a domain of multiple IP subnets without having to change their IP addresses. These methods, however, typically require complex IP-layer signaling and significant changes to the IP routers in the domain. Furthermore, these methods have not considered how to solve the data content synchronization problem.
From another viewpoint, in today's circuit-switched CDMA networks such as IS-95, a centralized Selection and Distribution Unit (SDU) is responsible for data distribution in the forward direction (from BS to MS). The SDU creates and distributes multiple streams of the same data over layer-2 circuits to multiple BSs that in turn relay the data to the MS. The MS's radio system (typically working below the IP layer) collaborates with the BSs to synchronize the radio channel frames and combine the radio signals received from different BSs to generate a single final copy of received data. The SDU helps ensure data content synchronization by ensuring that the matching layer-2 frames sent to different base stations contain copies of the same data. In the reverse direction (from MS to BS), the MS ensures that the matching layer-2 frames sent to different BSs contain copies of the same data. The SDU then selects one of the frames received from different base stations as the final copy of the data.
Accordingly, as evidenced by the foregoing discussion, achieving soft handoff among distributed iBSs introduces several new technical problems that cannot be solved readily by the mechanisms developed for today's centralized circuit-switched wireless networks.
One problem already alluded to is loss of data content synchronization. With distributed iBSs, centralized control entities, such as the SDU in circuit switched wireless networks, will no longer exist. Consequently, even though the CDMA radio system is capable of synchronizing the link and physical layer frames on the radio channel, it cannot, on its own, guarantee that the matching frames from different base stations will carry copies of the same data. For example, IP packets can be lost on their way to the MS, creating random gaps in the packet streams received by the MS from different iBSs. Furthermore, copies of the same data may arrive at the MS at different times due to the random delays suffered by the packets. Random gaps and delays can lead to a loss of data content synchronization. Suppose that packet X is lost at iBS 1 (due to, for example, buffer overflow) but is not lost at iBS 2. Then, another totally unrelated packet Y from iBS 1 and packet X from iBS 2 may arrive at the MS at the same time and the MS's radio system will not be able to tell that they are not copies of the same data and will hence erroneously combine X with Y.
Another problem is how to support soft handoff, which requires a mobile station to receive identical copies of the same data from multiple base stations simultaneously. When the mobile stations served by different base stations belong to different IP subnets, complex IP-layer signaling capabilities (e.g., IP multicast) have conventionally be required to direct copies of the same IP packets via multiple base stations to the mobile station. Furthermore, copies of the same IP packet arriving from different base stations to the mobile station will not be identical because these packets will carry different destination IP addresses. This makes it impossible for the mobile station's radio system to combine the signals from different base stations into a single copy of data.
The art is devoid of a methodology and concomitant systems that effect soft handoff in an all-IP wireless network that uses autonomous iBSs in a configuration having the following characteristics that differentiate the configuration from existing wireless networks: (a) the iBSs use IP protocols for both signaling and transport of user traffic. For example, they may route/forward IP packets based on information carried in the IP headers, perform IP-layer signaling, mobility management and Quality of Service (QoS) management functions; (b) the iBSs function autonomously. There is no centralized signaling and control over the behaviors of the iBSs; (c) the iBSs are interconnected via an IP network which could have arbitrary network topology such as bus, ring, star, tree, etc.; and (d) the cells (a cell is a geographical radio coverage area of a BS) can be arranged in any arbitrary configuration.